In the casting of steel, casting powder is applied continually to the surface of molten steel which is located in a casting mold. Here, the powder is melted by the heat to form a slag layer, which flows continuously into the gap between the casting mold and the solidified shell of the steel and is therefore consumed. The most important functions of the casting powder and/or the resultant slag are those of lubrication between the casting mold and the solidifying shell, control over the removal of heat from the hardening steel, the accommodation of deoxidation products, the prevention of reoxidation, and the heat insulation of the molten steel.
Typical casting powders for the continuous casting of steels are composed of a calcium silicate basis and customarily include a considerable amount of silicon dioxide (SiO2) and also other easily reducible components such as manganese oxide (MnO) and iron oxide (FeO), for example, and so, during the casting of aluminum-alloyed steels, the aluminum oxide content of the casting slag increases sharply because of a chemical reaction between the aluminum (Al) in the steel and the silicon dioxide and also manganese oxide and iron oxide in the casting slag:4Al+3SiO22Al2O3+3Si2Al+3MnOAl2O3+3Mn2Al+3FeOAl2O3+3Fe.
The typical takeup of aluminum oxide (Al2O3, also referred to as alumina) in the casting of Al-killed steels is around 2-4 wt %. With a higher aluminum content in the steel, there is an increase in the Al2O3 takeup in the casting slag. For example, in the production of TRIP steels with an Al content of around 1.2-1.5 wt %, the Al2O3 concentration in the casting slag rises to around 35-40 wt %.
The SiO2 content of the casting slag is reduced correspondingly, since silicon dioxide is reduced by aluminum. Generally speaking, this alters the properties of the casting slag.
For example, in view of the takeup of Al2O3 and the associated decrease in the fraction of SiO2, there are increases in the basicity, the viscosity, and the crystallization propensity of the casting slag, resulting in a deterioration in its lubricating effect. Generally speaking, amorphously solidifying casting slags have better lubricating effects than casting slags which solidify in crystalline form. In the prior art, slag systems based on calcium silicates are customarily used. These calcium silicate-based slag systems typically have solidification which is predominantly amorphous, as long as the aluminum fraction of the steel is not too high, i.e., in particular is <1 wt %. If Al contents in the melt are higher, however, the solidification of calcium silicate-based slag systems is predominantly crystalline.
In the casting of aluminum-alloyed steels which at the same time have a high manganese content (e.g. having Mn contents ≥15 wt % and Al contents ≥1 wt %), an additional complication to the casting process is that the liquidus temperature of these steels is around 100° C. lower than, for example, the liquidus temperature of the TRIP steels, because of the high Mn content. This means that for steels with high Al—Mn content, the melting and crystallization temperature of the casting slag must likewise be situated around 100° C. lower than for other steels of high Al content. Otherwise the casting slag may crystallize completely in the lower half of the casting mold, thereby eliminating its lubricating effect.
Because of the above-described changes in the properties of the casting slag during the casting of an Al-alloyed steel, the slag is frequently no longer able to fulfill its functions adequately or at all. Steels with a high aluminum content or aluminum and manganese content in particular, therefore, cannot be produced in an operationally reliable way with the casting powders known from the prior art.
Known from WO2011/090218 are casting powder compositions for the casting of steel which, however, in view of their high SiO2 content of 15-30 wt %, are unsuitable in particular for use in the casting of steel having a high aluminum content. The same also applies to the casting powders known from WO2007/148939 and their use in the casting of steel. JP 57184563 A discloses a powder for the coating of molten metal having a comparatively low SiO2 content. Because of its chemical composition, however, the slag obtained from this powder is unable to ensure the necessary functional qualities, such as lubricating effect and heat transfer, for example, especially for high Al—Mn steels. From experience, slags resulting from casting powders disclosed in JP 57184563 possess excessively high crystallization temperatures and excessively high viscosities in the operationally relevant range.